The present invention relates to providing an improved insulation for insulating double walled vessels for carrying cryogenic liquids. The improved light weight fiber glass insulation is used to fill the cavity between the walls providing high thermal resistivity thereby making the vessels ideal for use on over the road cryogenic tankers where the weight savings and good thermal performance provided by the insulation translates into transportation cost savings.
Large quantities of cryogenic liquids are routinely transported in over the road cryogenic tankers. Many of these tankers are insulated with expanded perlite at a density of 96 to 160 kg per cubic meter and reduced pressures below 0.07 millibar (0.05 mm Hg). Low binder content, 5-7 micron, fiber glass insulations at densities of 32 kg per cubic meter and at reduced pressures below 0.07 mb are also used to insulate these tankers. When compared to the expanded perlite insulation, the fiber glass insulations have significantly lower densities and offer similar thermal performances.
A few cryogenic tankers, particularly those used to transport lower temperature liquids, such as helium, use multi-layer insulations. These multi-layer reflective insulations are relatively expensive and comprise layers of aluminum foil or metalized film in the cavity between the walls of the vessel which are separated from each other to reduce heat transfer.
Insulation used in the vessels of cryogenic tankers should have the following properties: minimal weight and size to minimize tanker tare weight and maximize vessel capacity; efficient insulating properties to minimize cryogenic liquid boil-off rates; noncombustibility in an oxygen environment for use in vessels transporting liquid oxygen and liquid hydrogen; no outgasing or manageable outgasing during pump down to the vessel operating pressures; non-settling, even when subjected to the constant vibrations of over the road travel, so that noninsulated areas do not develop; and easy installation and maintenance over the life of the tanker.
Early work on fiber glass insulation for cryogenic applications was published in the following article: Christiansen, R. M., M. Hollingsworth, Jr., and H. N. Marsh, Jr. "Low-Temperature Insulating Systems," Advances in Cryogenic Engineering, 5:171-8 (1959). This article reports that "AA" fiber glass (0.8 to 1.3 micron fiber diameter) at densities from 8 to 240 kg per cubic meter demonstrated low thermal conductivities at reduced pressures and 191 degree K mean temperatures (with a 223 degree K delta T). The data indicates that increasing the density of the fiber glass insulation significantly reduces thermal conductivity with the optimum density being in the 128 to 240 kg per cubic meter range. The article also reported that the addition of reflective septa in the fiber glass insulation further reduced the thermal conductivity providing the greatest benefit on low density insulations.
Cryogenic thermal conductivity testing on larger diameter (9.0 micron) fiber glass insulations was reported in the following article: Smith, D. R. and J. G. Hust, "Effective Thermal Conductivity of Glass-Fiber Board and Standard Reference Materials," Thermal Conductivity 17, Plenum Press, 483-496 (1982). Smith tested SRM 1450a fiber glass board at a density of 127 kg per cubic meter and fiber glass blanket at a density of 14.75 kg per cubic meter. The data shows a large decrease in the thermal conductivity as the mean temperature decreases for the fiber glass blanket but less of an effect on the fiber glass board. According to the data reported by Smith, a 9.0 micron diameter fiber glass board at reduced pressures and a density of 127 kg per cubic meter would be an excellent insulation for cryogenic applications. However, at this density the fiber glass board would not provide any weight reduction for over the road cryogenic tankers where weight is directly related to operating costs.